During Shemoneh Esrei and Kiddush of Shavuos, we describe the Yom Tov as "this day of the holiday of Shavuos, the time of giving our Torah" (יום חג השבועות הזה זמן מתן תורתינו).
Before we had a fixed calendar, it was theoretically possible for any month to have either 29 or 30 days (unlike our current calendar wherein only Cheshvan and Kislev have variable lengths). If both Nissan and Iyar were to have 30 days, Shavuos (which is fixed at 50 days after the first day of Pesach, rather than being on a specific day on the calendar, as per Vayikra 23:16-21) would fall out on the 5th of Sivan. According to no opinion in the Gemara (Shabbos 86b) did the giving of the Torah occur on the 5th of Sivan.
What was said during davening and Kiddush of Shavuos those years? Did they still say "the time of giving our Torah," since it's the same time of year? Did they say something else ("the time of preparing for giving our Torah" maybe)? Did they say nothing at all?